The present invention relates generally to a technique for compressing relatively large light intensity ranges. More particularly, the invention discloses a twisted nematic liquid crystal cell with a photoconductor or photodiode applied field for use as a light intensity compressor for use with standard video or photographic equipment.
The human eye is capable of viewing objects under vastly differing illumination intensities. In contrast, when we attempt to record an image using photography and/or video camera recorders, we quickly find that most such devices can capture only a small part of the visual experience that we enjoy with our eyes. The problem of displaying scenes having large variations in interscene illumination intensity is twofold. First, commercially available cameras only accommodate interscene intensity variations of 100-500 times or 2-3 orders of magnitude. Secondly, the monitors available for such cameras accommodate interscene intensity changes over a range of approximately 50-100 times (1 or 2 orders of magnitude). The limited dynamic range of conventional video recording equipment has sponsored the need for a device capable of compressing the interscene intensity of images having a wide dynamic range to within the limited dynamic range presently available in conventional video recording equipment.
For example, the Applicant has found a need to view a crucible containing molten uranium in an atomic vapor laser isotope separation (AVLIS) system. However, this molten uranium containing crucible functions as a black body having a center which is at a temperature of approximately 4,000.degree. K. and having an outer temperature on the order of 1,200.degree. K. Thus, the temperature range across this black body is approximately 3,000.degree. K. and has a correspondingly large interscene light intensity range of approximately 7 orders of magnitude at 480 nm. Such a large interscene intensity range is far too large for a standard video camera or similar instruments to receive and decipher. Therefore, if standard viewing and/or recording equipment is to be used to view the crucible, its light intensity range must be compressed to a tolerable level.
Complete control over the interscene intensity range is possible using two dimensional light valves (TDLV). The simplest form of TDLV is photo grey glass, which can compress the interscene intensity dynamic range by at least a factor of 10. Alternatively, prior art techniques have attempted to accomplish the same purpose electronically. That is, by converting the incoming light to corresponding electrical signals, compressing the electrical signals, and then converting those compressed electrical signals back to light. The resultant light is thus compressed relative to the incoming light. However, Applicant has found that such "electronic" approaches to light compression are relatively complicated and expensive.
An article written by Younse, et al. entitled "Wide Dynamic Range CCD Camera," SPIE Volume 501: State-of-the-Art Imaging Arrays and their applications (1984), discusses the use of a liquid crystal attenuator operated as a variable neutral density filter for extending the dynamic range of a solid state television camera by an order of magnitude. As described therein, a twisted nematic liquid crystal is sandwiched between a pair of crossed polarizers. An electrode pattern formed of In.sub.2 O.sub.3 disposed between the top polarizer and the liquid crystal is adapted to apply an electric field in the vicinity of the liquid crystal cell. An ambient photo sensor controls the intensity of the field applied as a direct function of the ambient light level. While such a device is capable of extending the dynamic range of a CCD imager by a factor of twelve, it provides substantially equal compression over the entire image and thus is inappropriate for recording scenes having large interscene light intensity variations. Rather it functions substantially as an electrical equivalent of an auto-iris lens.
An article by Marqerum, et al., in Applied Physics Letters, Volume 17(2) (1970) entitled "Reversible ultraviolet imaging with liquid crystals," describes a method for recording light-scattering images in nearly real-time using nematic liquid crystals. The device described therein includes a nematic liquid crystal that is covered with a photoconductive coating of ZnS which is sensitive to ultraviolet light. The liquid crystal and photoconductive layers are sandwiched between a pair of transparent electrodes which are used to apply a voltage to the device. The actual intensity of the field in the region of the liquid crystal will depend upon the extent to which the photoconductive layer has been activated. Ultraviolet exposure of the photoconductive ZnS layer is used to regulate the field applied in the liquid crystal region. However, the device described therein in not suitable for use as a light intensity compressor since it is designed to store photo images within the liquid crystal layer.
Co-pending application Ser. Nos. 863 912, now U.S. Pat. No. 4,815,828, filed May 16, 1986 and 863,758, now U.S. Pat. No. 4,726,660, filed May 16, 1986 disclose a wide variety of pin hole camera assemblies that incorporate devices for compressing light intensity ranges utilizing a variety of optical compression techniques including photogray glass, interference filters and thermally responsive liquid crystal notch filters. The present invention relates to these two application and presents an alternative method of optically compressing light intensity using a photoactivated twisted nematic liquid crystal arrangement.